Why are all my Chat Messages Squelched?

When I attempt to send a message to a player on my Battle.net client's Friends list, a red dialogue is returned with the following:

"! Your message to [name] wasn't delivered because you are squelched."

I attempted sending messages to another dozen or more players on my Friends list and each attempt resulted with the same "! Your message [name] wasn't delivered because you are squelched." I find it highly unlikely that players whom I have been corresponding with for months, and even years, have all at once decided to squelch me.

When I attempt to message someone in-game (in Hearthstone), the message appears, the chat bubble is displayed, and it appears as if the message has been sent -- however, there is no subsequent reply.

Blizzard had not sent me any notification via e-mail or otherwise in regard to a possible penalty or sanction in this regard.